This invention relates generally to a device for compensation of chromatic dispersion in optical fiber communication systems and specifically to a multiple pass multi-cavity etalon dispersion compensating device.
Most high-speed fiber optic communication systems today use externally modulated lasers to minimize laser xe2x80x98chirpxe2x80x99 and reduce the effects of chromatic dispersion in the fiber. Even with external modulation, there is a certain amount of xe2x80x98chirpxe2x80x99 or broadening of the laser spectrum, because any modulated signal must contain frequency xe2x80x98sidebandsxe2x80x99 which are roughly as wide as the modulation rate. Higher bit rate transmission systems consequently have broader frequency sidebands, and at the same time can tolerate less phase delay because of the shorter bit period. Next-generation high bit rate systems are consequently very sensitive to chromatic dispersion of the optical fiber and any components such as WDM""s within the system.
Chromatic dispersion of optical fiber is roughly constant over the 1550 nm communication window, and can be compensated by several techniques including dispersion compensating fiber, fiber Bragg gratings, etc. However, certain wavelength filtering components such as WDM""s can have significant dispersion characteristics due to a fundamental Kramers-Kronig type relationship between transmission spectrum and dispersion characteristics. This type of dispersion characteristic typically varies substantially over the narrow WDM passband, and therefore is difficult to compensate using conventional techniques such as dispersion compensating fiber. It is one objective of the present invention to compensate for the dispersion from WDM devices, including multiplexers, demultiplexers, and interleavers. Conventional laser systems are known to utilize directly modulated semiconductor lasers. In combination with chromatic dispersion characteristics of single-mode optical fiber, chirping of these lasers contributes to the spread of optical pulses and results in intersymbol interference and overall degradation in transmission. Intersymbol interference in a digital transmission system is the distortion of the received signal by overlap of individual pulses to the degree that the receiver cannot reliably distinguish between groupings of pulses.
Current and xe2x80x9cnext-generationxe2x80x9d high speed systems employ transmitters which use narrow linewidth lasers and external modulators in a window or range of wavelengths about 1550 nm. These external modulators generally have a very low chirp; some designs have a zero or negatively compensating chirp. Nevertheless, transmission distance is still dispersion-limited to about 80 kilometers at transmission rates of 10 Gb/s using conventional single mode fibers.
One solution to this problem is in the use of dispersion shifted fiber which has little dispersion in the 1550 nm window. However, replacing existing fiber with dispersion shifted fiber is costly. Other approaches have been proposed such as optical pulse shaping to reduce laser chirp, using a semiconductor laser amplifier to impose a chirp on the transmitted signal that counteracts the chirping of the directly modulated semiconductor laser.
Approaches that are more consistent with the teachings of this invention attempt to reduce the intersymbol interference at or near the receiver, or intermediate the transmitter and the receiver. Essentially any medium capable of providing a sufficient dispersion opposite to that of the optical fiber can serve as an optical pulse equalizer. For example it is known to use a special optical fiber having an equal chromatic dispersion at a required operating wavelength but opposite in sign to that of the transmitting fiber. Other methods include the use of fiber Bragg gratings, FBGs, as disclosed in U.S. Pat. No. 5,909,295 in the name of Li et al., and disclosed by Shigematsu et al., in U.S. Pat. No. 5,701,188 assigned to Sumitomo Electric Industries, Ltd., and the use of planar lightwave circuit (PLC) delay equalizers. Unfortunately, special compensating fiber has a very high insertion loss and in many applications is not a preferable choice. Fiber gratings are generally not preferred for some field applications due to their narrow bandwidth, and fixed wavelength. PLCs are also narrow band, although tunable devices; fabricating a PLC with large dispersion equalization remains to be difficult. Shigematsu et al. disclose a hybrid of both of these less preferred choices; dispersion compensating fiber with chirped Bragg gratings.
In a paper entitled xe2x80x9cOptical Equalization to Combat the Effects of Laser Chirp and Fiber Dispersionxe2x80x9d published in the Journal of Lightwave Technology. Vol. 8, No. 5, May 1990, Cimini L. J. et al. describe an optical equalizer capable of combating the effects of laser chirp and fiber chromatic dispersion on high-speed long-haul fiber-optic communications links at 1.55 xcexcm. Also discussed is a control scheme for adaptively positioning the equalizer response frequency. Cimini et al. describe a device having only one common input/output port at a first partially reflective mirror and a second 100% reflective mirror together forming a cavity. The control scheme described attempts to track signal wavelength by obtaining feedback from a receiver. The amplitude response of the equalizer is essentially flat with wavelength at the input/output port, and thus, the proposed control scheme is somewhat complex requiring processing of high speed data at the optical receiver. As well, the proposed control method is stated to function with return to zero, RZ, signals but not with non-return to zero, NRZ, signals, a more commonly used data format. Although the equalizer described by Cimini et al. appears to perform its intended basic dispersion compensating function, there exists a need for an improved method of control of the position of the equalizer frequency response, and as well, there exists a need for an equalizer that will provide a sufficient time shift over a broader range of wavelengths. U.S. Pat. No. 5,023,947 in the name of Cimini et al., further describes this device.
A Fabry-Perot etalon having one substantially fully reflective end face and a partially reflective front face is known as a Gires-Tournois (GT) etalon. In a paper entitled xe2x80x9cMultifunction optical filter with a Michelson-Gires-Tournois interferometer for wavelength-division-multiplexed network system applicationsxe2x80x9d, by Benjamin B. Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device is described which is hereafter termed the MGT device. The MGT device as exemplified in FIG. 1 serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror 16. The etalon 10 used has a 99.9% reflective back reflector 12r and a front reflector 12f having a reflectivity of about 10%; hence an output signal from only the front reflector 12f is utilized.
In an article entitled xe2x80x9cOptical All-Pass Filters for Phase Response Design with Applications for Dispersion Compensationxe2x80x9d by C. K. Madsen and G. Lenz, published in IEEE Photonics Letters, Vol. 10 No. 7, July 1998, a coupled reflective cavity architecture in optical fiber is described for providing dispersion compensation. Cavities are formed in the optical fiber between fiber Bragg grating reflectors. However a multi-cavity filter in fiber has a limited free spectral range (FSR) insufficient for a telecommunications system. For a typical 100 GHz FSR required in the telecommunications industry, the cavity length is about 1 mm. A Bragg grating reflector, if manufactured using commonly available grating-writing techniques, would need to be longer than 1 mm, and hence the two reflector cavity structure would be too long to achieve the necessary FSR. Another draw back to this prior art solution is the requirement for an expensive optical circulator to separate the input and output signals.
As of late, interleaving/de-interleaving circuits are being used more widely. These specialized multiplexor/demultiplexers serve the function of interleaving channels such that two data streams, for example a first stream consisting of channel 1, 3, 5, 7, and so on, is interleaved, or multiplexed with a second stream of channels, 2, 4, 6, 8, and so on, for forming a single signal consisting of channels 1, 2, 3, 4, 5, 6, 7, 8, and so on. Of course the circuit can be used oppositely, to de-interleave an already interleaved signal, into plural de-interleaved streams of channels. One such interleaver circuit is described in U.S. Pat. No. 6,125,220 issued in the name of Copner et al., and another is in U.S. Pat. No. 6,040,932 issued in the name of Colbourne et al. Although interleaver circuits perform a desired function, it has been discovered that some of these circuits suffer from unwanted periodic chromatic dispersion within each channel. It is this type of periodic dispersion that can be obviated or lessened by the instant invention. It should also be noted that in many instances it is not desirable to completely eliminate all chromatic dispersion; it is believed that a small amount of such dispersion can be useful in reducing non-linear effects in WDM systems; therefore, the instant invention can be used to lessen dispersion by a required amount.
Hence, it is an object of this invention to overcome some of the limitations of the prior art described above. Furthermore, it is an object of the invention to provide a device that will compensate for or lessen dispersion over a plurality of interspaced wavelength channels simultaneously.
In accordance with the invention, there is provided a dispersion compensation device for compensating a dispersion of an optical input beam, the device comprising:
polarization dependent beam routing means having an input port, for routing a polarized optical beam launched into the input port along a first path in one of two directions in dependence upon the polarization state of the polarized optical beam,
at least one multi-cavity etalon defining at least two resonant cavities, optically coupled to the routing means, said at least one multi-cavity etalon for receiving at least one optical beam from the routing means and for directing at least one reflected optical beam back to the routing means for the reflected light beam to follow a second path in the routing means; and
at least one rotator for rotating the polarization of light in the optical path between the routing block and the etalon so that the at least one reflected light beam follows the second path in the routing means,
whereby the polarized light beam launched into the input port undergoes multiple reflections from the etalon to reduce dispersion of the optical input beam.
The device may also comprise beam directing means, optically coupled to the routing means, for receiving at least one beam from the routing means and for directing at least one beam back to the routing means.
The routing means may be exemplified by a birefringent crystal or by a polarizing beam splitter.
If the input beam is not polarized, a polarization diversity means should be provided to divide the optical input beam into two sub-beams having orthogonal polarizations before the sub-beams are passed into the routing means. After the multiple passes and reflections from the etalon, the two sub-beams may be re-combined to recover the full power of the dispersion-compensated optical beam.
In accordance with the another aspect of the invention, there is provided a dispersion compensation device for compensating a dispersion of an optical input beam comprising:
input beam splitting means for spatially separating the input beam into two orthogonally polarized beams;
first polarization rotating means, optically coupled to the input beam splitting means, said first polarization rotating means for rotating the polarization of one of the two beams such that the two beams have the same polarization;
polarization dependent beam routing means, optically coupled to the first polarization rotating means, said polarization dependent beam routing means for routing the two beams on a first path for the two beams having a first polarization and on a second path for the two beams having a second polarization, orthogonal to the first polarization;
second polarization rotating means, optically coupled to the polarization dependent beam routing means, said second polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization;
at least one multi-cavity etalon, optically coupled to the second polarization rotating means, said at least one multi-cavity etalon for receiving the two beams from the second polarization rotating means and for launching the two beams back to the second polarization rotating means, said multi-cavity etalon having at least one end face that is highly reflective and substantially not transmissive to light and at least two other faces that are partly reflective and partly transmissive, the one end face and the at least two other faces being separated from one another by predetermined gaps, the at least three faces forming at least two resonant cavities;
third polarization rotating means, optically coupled to the polarization dependent beam routing means, said third polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization;
beam directing means, optically coupled to the third polarization rotating means, said beam directing means for receiving the two beams from the third polarization rotating means and for directing the two beams back to the third polarization rotating means;
fourth polarization rotating means, optically coupled to the polarization dependent beam routing means, said fourth polarization rotating means for rotating the polarization of one of the two beams such that the two beams have orthogonal polarizations; and
output beam combining means, optically coupled to the fourth polarization rotating means, said output beam combining means for spatially combining the two orthogonally polarized beams into an output beam;
whereby the two beams undergo multiple passes through the at least one multi-cavity etalon and thereby the dispersion correction of the two beams is increased.
It is understood by those educated in this art that, for example, the splitting and combining means could be birefringent crystals with a walk-off axis orthogonal to the walk-off axis of the routing means which could be another birefringent crystal. Further, the walk-off axes of both of these elements are orthogonal to the optical path through these elements.
In accordance with the another aspect of the invention, there is provided a dispersion compensating system, containing at least two dispersion compensating devices, for compensating an overall dispersion of an optical input beam comprising:
input beam routing means for routing an input beam to a first dispersion compensating device, said first dispersion compensating device comprising:
input beam splitting means for spatially separating the input beam into two orthogonally polarized beams;
first polarization rotating means, optically coupled to the input beam splitting means, said first polarization rotating means for rotating the polarization of one of the two beams such that the two beams have the same polarization;
polarization dependent beam routing means, optically coupled to the first polarization rotating means, said polarization dependent beam routing means for routing the two beams on a first path for the two beams having a first polarization and on a second path for the two beams having a second polarization, orthogonal to the first polarization;
second polarization rotating means, optically coupled to the polarization dependent beam routing means, said second polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization;
at least one multi-cavity etalon, optically coupled to the second polarization rotating means, said at least one multi-cavity etalon for receiving the two beams from the second polarization rotating means and for launching the two beams back to the second polarization rotating means, said multi-cavity etalon having at least one end face that is highly reflective and substantially not transmissive to light and at least two other faces that are partly reflective and partly transmissive, the one end face and the at least two other faces being separated from one another by predetermined gaps, the at least three faces forming at least two resonant cavities;
third polarization rotating means, optically coupled to the polarization dependent beam routing means, said third polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization;
beam directing means, optically coupled to the third polarization rotating means, said beam directing means for receiving the two beams from the third polarization rotating means and for directing the two beams back to the third polarization rotating means;
fourth polarization rotating means, optically coupled to the polarization dependent beam routing means, said fourth polarization rotating means for rotating the polarization of one of the two beams such that the two beams have orthogonal polarizations;
output beam combining means, optically coupled to the fourth polarization rotating means, said output beam combining means for spatially combining the two orthogonally polarized beams into an output beam;
whereby the two beams undergo multiple passes through the at least one multi-cavity etalon and thereby the dispersion correction of the two beams is increased;
at least one intermediate beam routing means for routing an output beam, of at least a first dispersion compensating device, such that said output beam becomes an input beam of another dispersion compensating device, said another dispersion compensating device comprising:
input beam splitting means for spatially separating the input beam into two orthogonally polarized beams;
first polarization rotating means, optically coupled to the input beam splitting means, said first polarization rotating means for rotating the polarization of one of the two beams such that the two beams have the same polarization;
polarization dependent beam routing means, optically coupled to the first polarization rotating means, said polarization dependent beam routing means for routing the two beams on a first path for the two beams having a first polarization and on a second path for the two beams having a second polarization, orthogonal to the first polarization;
second polarization rotating means, optically coupled to the polarization dependent beam routing means, said second polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization;
at least one multi-cavity etalon, optically coupled to the second polarization rotating means, said at least one multi-cavity etalon for receiving the two beams from the second polarization rotating means and for launching the two beams back to the second polarization rotating means, said multi-cavity etalon having at least one end face that is highly reflective and substantially not transmissive to light and at least two other faces that are partly reflective and partly transmissive, the one end face and the at least two other faces being separated from one another by predetermined gaps, the at least three faces forming at least two resonant cavities;
third polarization rotating means, optically coupled to the polarization dependent beam routing means, said third polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization;
beam directing means, optically coupled to the third polarization rotating means, said beam directing means for receiving the two beams from the third polarization rotating means and for directing the two beams back to the third polarization rotating means;
fourth polarization rotating means, optically coupled to the polarization dependent beam routing means, said fourth polarization rotating means for rotating the polarization of one of the two beams such that the two beams have orthogonal polarizations;
output beam combining means, optically coupled to the fourth polarization rotating means, said output beam combining means for spatially combining the two orthogonally polarized beams into an output beam;
whereby the two beams undergo multiple passes through the at least one multi-cavity etalon and thereby the dispersion correction of the two beams is increased; and
output beam routing means, optically coupled to the last dispersion compensating device, said output beam routing means for routing an output beam of the last dispersion compensating device to an output port;
whereby the beam undergoes dispersion correction at each dispersion compensating device that results in balancing the compensation of the overall dispersion of an optical input beam.
An additional embodiment of this invention is provided by a method of dispersion compensation for simultaneously compensating for dispersion present within individual channels in a multi-channel signal, the method comprising:
providing a polarization dependent beam routing and directing means for routing and directing the multi-channel signal in a polarization dependent manner and at least one multi-cavity etalon optically coupled to said polarization dependent beam routing means; and
launching a multi-channel signal into said polarization dependent beam routing means to allow for multiple passes through said beam routing means and said multi-cavity etalon, and capturing a dispersion compensated multi-channel signal from said polarization dependent beam routing means.